This application claims the benefit of Korean Patent Application No. 1999-11516, filed on Apr. 1, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
(1) Field of the Invention
The present invention relates to an X-ray image sensor, and more particularly to an X-ray image sensor fabricated utilizing a TFT (Thin Film Transistor) array process and a method for fabricating the same.
(2) Description of Related Art
An X-ray detection method that has been widely used for medical diagnosis is such that an X-ray detecting film is used to produce a photograph and some predetermined printing procedures are required to obtain the result.
However, digital X-ray detectors employing TFT (Thin Film Transistor) have been developed recently due to the development of semiconductor technology. This X-ray image sensor has an advantage that a real time diagnosis can be obtained immediately after photographing because it uses a TFT as switching element.
FIG. 1 is a schematic cross-sectional view illustrating the structure and operation of an X-ray image sensing device 100 which comprises a lower substrate 1, a thin film transistor 3, a storage capacitor 10, a pixel electrode 12, a photoconductive film 2, a protection film 20, a conductive electrode 24 and a high voltage D.C. (direct current) power supply 26.
Photoconductive film 2 produces internal electric signals, i.e. electron-hole pairs in proportion to the strength of external signals such as incident electromagnetic waves or magnetic waves. That is, the photoconductive film 2 serves as a converter to detect external signals, particularly X-rays and convert them into electric signals. Either electrons or holes are gathered at pixel electrode 12 located beneath the photoconductive film 2 depending on a voltage (Ev) applied to the conductive electrode 24 by the high voltage D.C. power supply 26, and then are stored in storage capacitor 10 formed in connection with a ground line grounded externally. Charges stored in the storage capacitor 10 are transferred to the TFT 3, controlled externally, to an external image display device and form X-ray images.
In an X-ray image sensing device, to detect and convert even the weakest X-ray into electric charges, it is required to decrease the trap state density for the electric charge in the photoconductive film 2 and to decrease the amount of current in the non-vertical direction by applying a relatively high voltage (more than 10V/xcexcm) in the vertical direction between the conductive electrode 24 and pixel electrode 12.
Electric charges in the photoconductive film 2 produced by X-ray energy are trapped and gathered on the pixel electrode 12. Even during the OFF state of the TFT 3, electric charges trapped and gathered on the pixel electrode 12, particularly over the channel region of the TFT 3, induce a potential difference between the TFT 3 and the pixel electrode, which causes the same effect in the TFT 3 as ON state. That is, the pixel electrode 12 functions as a gate of the TFT 3, thus, adversely affecting the switching operation of the TFT 3 and increasing leakage current through the TFT 3 even when in an OFF state. This results in an undesired image.
FIG. 2 is a cross-sectional view schematically illustrating a conventional X-ray image sensor. U.S. Pat. No. 5,498,880 discloses one kind of structure wherein the pixel electrode 12 extends to cover the upper part of TFT 3 (a so called xe2x80x9cmushroom structurexe2x80x9d) to prevent the trapping of electric charges in the upper part of TFT 3 induced from the electric charges produced in photoconductive film 2 by X-ray energy.
The manufacture of the conventional X-ray image sensor will be described hereinafter referring to FIG. 2.
First, substrate 1 is deposited with a metal and patterned to form a gate electrode 31. Then, SiNx is deposited thereon in a thickness of about 100 nm to form a first insulation film 34a. After the formation of film 34a, a transparent conductive material is deposited and patterned to form a first capacitor electrode 40. ITO (indium tin oxide) is most commonly used as the transparent conductive material.
After forming the first capacitor electrode 40, a second insulation film 34b is formed on the first insulation film 34a while covering the first capacitor electrode 40. At a predetermined position of the second insulation film 34b on the first capacitor electrode 40, a contact hole 41 is formed for contact with a ground line 42 that will be formed later. Thereafter, a source/drain metal material is deposited and patterned to form a source electrode 33, a drain electrode 32 and the ground line 42. The source/drain metal is usually aluminum that has a low resistance and good deposition properties. Protection film 46 is formed after the formation of 33, 32 and 42, in order to protect TFT 3 from external impact or humidity.
In the protection layer 46 on the source electrode 33, contact holes are formed for contact with the pixel electrode that will be formed later. Then, the protection layer 46 formed in the upper part of the first capacitor electrode 40, except on ground line 42, is etched out in order to decrease the thickness of the dielectric layer and increase the capacity of the storage capacitors. Then, ITO is deposited and patterned to form a pixel electrode 12 which serves as second capacitor electrode, and a photoconductive film 2 is formed by deposition over the whole substrate 1. The later procedures are abbreviated here.
In an X-ray image sensor adopting the so-called xe2x80x9cmushroom structurexe2x80x9d as described above, electric charges produced by X-ray energy gather on the pixel electrodes and there is formed a parasitic capacitor between pixel electrode and TFT.
The capacity of a parasitic capacitor has an inverse relationship with respect to the thickness of the protection film for protecting the channel part in the upper part of TFT such that it increases as the thickness is decreased, inducing a large amount of charges to the channel part, which increases the amount of leakage current even if TFT is in an xe2x80x9coffxe2x80x9d state and deteriorates its switching operation.
Though the capacity of the parasitic capacitor of the TFT may be decreased when the thickness of the protection film made of acrylic on the TFT is increased, in the above structure there is a limit to reduce the capacity of the parasitic capacitor, since the dielectric constant of acrylic is relatively high.
Furthermore, the second insulation film used as a dielectric of the storage capacitor is formed to be thin, with a thickness of about 200 nm in a conventional X-ray image sensor. Therefore the second insulation film may be etched out or overetched while etching the organic insulation film as a protection film in order to form a contact hole for contacting the pixel electrode, or the second capacitor electrode and the second insulating film to decrease the thickness of the dielectric layer. This causes an electrical short of the first and second capacitor electrodes and accordingly the number of point defects is increased, leading to low yield. The present invention has been developed as a result of the continuous research done by the inventors for solving the above-described problems.
An object of the present invention is to provide an X-ray image sensor in which the xe2x80x9coffxe2x80x9d electric current is decreased by lowering the leakage current of a TFT.
Another object of the present invention is to provide an X-ray image sensor which can prevent electrical shorts between gate and source/drain electrodes and a capacitor electrode.
A further object of the invention is to provide an X-ray image sensor which can decrease processing errors which may occur in the production process.
In order to achieve the above objects, the present invention provides, in a first aspect, an X-ray image sensor which comprises: a photoelectric conversion part affecting electric charges in accordance with a received amount of X-ray energy; a pixel electrode for collecting the electric charges; a storage capacitor for storing the electric charges collected in the pixel electrode, having a first capacitor electrode, a dielectric layer deposited on the first capacitor electrode, and a second capacitor electrode on the dielectric layer, the second capacitor electrode contacting the pixel electrode through a first contact hole formed in a protection film on the second capacitor electrode; and a switching part controlling release of electric charges stored in the storage capacitor to an external circuit.
In a second aspect, the present invention also provides an X-ray image sensor which comprises: a photoelectric conversion part affecting electric charges in accordance with received amount of X-ray; a pixel electrode for collecting the electric charges; a storage capacitor for storing the electric charges collected in the pixel electrode; and a thin film transistor for controlling release of electric charges stored in the storage capacitor to an external circuit, having gate, source and drain electrodes, wherein the pixel electrode extends over the thin film transistor, and a protection film made of benzocyclobutene lies therebetween.
In a third aspect, the present invention also provides a method for fabricating an X-ray image sensor, comprising the steps of: forming a gate electrode on a substrate; forming a first insulation film on the substrate while covering the gate electrode; forming a source island and first capacitor electrode on predetermined positions of the first insulation film; forming an active layer on the first insulation film over the gate electrode so that the active layer overlaps one end of the source island; forming on the active layer, a drain electrode and a source electrode contacting a portion of the source island and forming a ground line on the first capacitor electrode; depositing a second insulation film on the first capacitor electrode while covering the ground line and the source and drain electrodes; forming a second capacitor electrode on the second insulation film over the first capacitor electrode; depositing an organic protection film on the second insulation film and on the second capacitor electrode; forming contact holes by etching the organic protection and second insulating films so that a portion of the source island and source electrode and a portion of the second capacitor electrode are exposed; forming on the organic protecting film a pixel electrode in contact with the source island and source electrode, and the second capacitor electrode though the contact holes, forming a photoconductive film on the pixel electrode; and forming a conductive electrode on the photoconductive film.
In a fourth aspect, the present invention provides a method for fabricating an X-ray image sensor, comprising the steps of: forming a gate electrode on a substrate; forming a first insulation film on the substrate while covering the gate electrode; forming an active layer on the first insulation film over the gate electrode; forming a drain electrode and a source electrode on the active layer and a grounding line on the first capacitor electrode; forming a source island overlapping the source electrode and a first capacitor electrode covering the ground line on the first insulating film; depositing a second insulation film on the first capacitor electrode while covering the ground line and the source and drain electrodes; forming a second capacitor electrode on the second insulation film over the first capacitor electrode; depositing an organic protection film on the second insulation film and on the second capacitor electrode; forming contact holes by etching the organic protection and second insulating films so that a portion of the source island and source electrode and a portion of the second capacitor electrode may be exposed; forming on the organic protecting film; a pixel electrode in contact with the source island and source electrode and the second capacitor electrode though the contact holes, forming a photoconductive film on the pixel electrode; and forming a conductive electrode on the photoconductive film.
The area of the first capacitor electrode is larger than that of the second capacitor electrode and vice versa.
The second capacitor electrode of the storage capacitor is made of transparent conducting material.
The transparent conducting material for the second capacitor electrode is indium tin oxide.
The switching part is a thin film transistor having gate, source and drain electrodes, the source electrode contacting the pixel electrode through a source island, thereby decreasing contact resistance between the source electrode and the pixel electrode.
The source island is made of transparent conducting material.
The transparent conducting material for the source island is indium tin oxide.
The protection film is an organic insulation film.
The organic insulation film is made of a material selected from the group consisting of BCB (benzocyclobutene), acrylic and polyamide.
The storage capacitor further includes a grounding line contacting the first capacitor electrode in order to reset the storage capacitor.
The pixel electrode contacts the second capacitor electrode through a second contact hole in the protection film over the grounding line.
The grounding line is located on the first capacitor electrode of the storage capacitor and vice versa.
The gate electrode of the thin film transistor is two-layered.
The lower layer for the gate electrode is made of aluminum or aluminum alloy and a upper layer for the gate electrode is selected from a group consisting of molybdenum(Mo), tantalum(Ta), tungsten(W), niobium(Nb) and antimony(Sb).
The pixel electrode is formed on the organic protecting film while extending over the active layer.